1. Field of the Invention
The present invention relates to an on-chip thin film Zernike phase plate for releasing charging, and applications thereof.
2. Description of the Related Art
Organic devices, such as polymer solar cells and organic light-emitted diodes (OLED), have attracted considerable attention in recent years. The efficiencies of these organic devices greatly depend on their nanoscale structures. For example, the nanoscale morphology affects the charge transport in the donor/acceptor blend, and hence influences the power conversion efficiency of a bulk heterojunction polymer solar cell.1-5 Characterizing nano-structures in organic devices thus has a critical importance in optimizing their performance. Transmission electron microscopy (TEM) has long been used for characterizing nano/micro-structures in materials, while its application on organic materials has been limited because the contrast of TEM images for organic specimens is usually poor since they are constituted with light elements, i.e. C, O, and N. The defocus technique is well known for enhancing contrast of TEM images, while it is a tricky technique for not only does it result in a significant loss of resolution, but it also causes some artifacts, consequently complicating the interpretation of the images. In-focus TEM imaging for organic materials with enhanced contrast is therefore a much needed technique, particularly with the rapidly growing interest in organic devices.
Adopting a phase plate in the optical path of a TEM to enhance the contrast of the in-focus image was proposed 60 years ago.6,7 Various TEM phase plates have been in development for decades,8-16 and some promising results using thin film Zernike phase plates have been presented.17,18 Using a thin film phase plate inevitably scarifies some high resolution signals, due to the loss of coherence resulting from the interaction between the scattered wave and the carbon film. Such a drawback, however, does not preclude its major advantage in characterizing nano-structures of organic materials which are too vague under conventional TEM. Although Zernike phase plates have been successfully used to reveal the structure of ice-embedded biomolecules,18 a more reliable phase plate technique that allows phase contrast images to be taken reliably and repeatedly is still in need, since the performance of TEM phase plates deteriorates too quickly.7,13,16 One of the major causes for the unstable performance of a TEM phase plate is the charging effect.13,16 When a physical phase plate is present in the pathway of the electron beam, the build up of charged particles in some local sites would result in instability and distortion of the image formed. Such a notorious charging effect resulting from phase plates has been known for some time, and is still considered an obstacle for TEM phase plate technology to be put into routine use until very recently.13,16 Practically, the investigation of nanostructures of organic materials with phase plates can hardly be realized without a reliable and routinely achievable method for taking in-focus phase TEM images.